This invention relates to shunt valves for venting cerebrospinal fluid ("CSF") in the treatment of hydrocephalus and similar conditions of impaired circulation and absorption of body fluids.
Cerebrospinal fluid shunt valves have been in use for over twenty years. Broadly speaking, they function by venting excess cerebrospinal fluid from the brain into the venous system or other receptive cavities (e.g., peritoneal, pleural). Many such valves, including the earliest designs, operate by controlling the amount of fluid flow. The neurosurgeon makes an estimate of the amount of flow required to relieve the hydrocephalus and selects a valve of that flow capacity. The selection is made difficult by the wide variation in normal flow rates.
About twenty years ago, applicant Salomon Hakim developed an altogether different valve, one that controlled intraventricular pressure rather than flow. That valve, which is today known as the Cordis-Hakim shunt valve, and which is described in U.S. Pat. No. 3,288,142, has been enormously successful and remains, even today, one of the most popular shunt valves in use. It has a spherical sapphire ball biased against a conical valve seat by a stainless steel spring. The pressure of cerebrospinal fluid pushes against the sapphire ball and spring in a direction tending to raise the ball from the seat. When the pressure difference across the valve (e.g., the pressure difference between the cerebral ventricle and the drainage site) exceeds a so-called popping pressure, the ball rises from the seat to vent cerebrospinal fluid. As the flow rate through the valve increases, the ball moves further away from the seat to provide a larger valve orifice, one that is always large enough that the pressure drop across the orifice never rises much above the popping pressure. Accordingly, the differential pressure across the valve remains nearly constant for any flow rate encountered within the cerebrospinal fluid system.
As successful as the Cordis-Hakim valve has been, it has one important limitation. It can only provide a fixed popping pressure. In treating hydrocephalus, it is often desirable to vary the popping pressure in accordance with ventricle size and treatment objective. For example, initial treatment may require a lower than normal pressure to initiate shrinkage of the ventricles, but as the ventricles decrease in size, the popping pressure should be increased gradually so that when the ventricles return to normal size the intraventricular pressure is at its normal value and the intracranial force systems are in balance (i.e., the popping pressure is set at a level that will stabilize the ventricles at a desired size). Generally speaking, the popping pressure should be varied inversely with variation in ventricle size. There is a danger, if a low pressure valve is left in a patient, of the ventricles collapsing to a condition known as "slit" ventricles. A fuller discussion of these matters can be found in Hakim et al., "A Critical Analysis of Valve Shunts Used in the Treatment of Hydrocephalus", Developmental Medicine and Child Neurology, Vol. 15, No. 2, Apr. 1973, pp. 230-255.
A further reason for providing adjustability in popping pressure is to correct for the wide variation in nominal popping pressure typical in manufactured valves. For example, a valve with a nominal popping pressure of 100 mm may have a much larger actual pressure. Even if that variation is discovered prior to surgery, there is nothing that can be done to correct the difficulty except to replace the valve with another, if one is available.
Pressure adjustment also has advantages in the manufacture of shunt valves. Popping pressure can be set more accurately after assembly and prior to shipment.
This adjustment capability could theoretically be achieved by modifying the original Hakim valve as shown in FIG. 5 of Hakim, "Hydraulic and Mechanical Mis-matching of Valve Shunts Used in the Treatment of Hydrocephalus: the Need for a Servo-valve Shunt", Developmental Medicine and Child Neurology, Vol. 15, No. 5, Oct. 1973, pp. 646-653, but the modified valve is not practical.